Data transmission systems in which data obtained using artificial satellites such as observation satellites operating in the earth orbit in the outer space is transmitted from the artificial satellites or the like to receiving stations on the Earth have been introduced. In recent years, with an increase in the precision of observation equipment mounted in the observation satellites, the amount of data transmitted by the observation satellites has been increasing. Thus, in order to transmit the data at higher speeds than ever before, the data transmission systems using a band of 26 GHz or the like in which a wide band is available are being studied. Hereinafter, artificial satellite is abbreviated as satellite.
In the data transmission systems using the satellites, signal attenuation such as rain attenuation due to rain, snow, or the like occurs when the receiving stations receive the signals transmitted from the satellites. In particular, as the frequency band becomes higher, the amount of attenuation of signals due to rain increases. Thus, various methods are being studied as measures against the rain attenuation. The measures against the rain attenuation include, for example, the site diversity using a plurality of the receiving stations and the adaptive modulation.
The site diversity using a plurality of the receiving stations is, as described in Non Patent Literature 1, for example, a method in which a plurality of the receiving stations disposed on the Earth receives the data transmitted by the transmitting station mounted in the satellite, and the central station receives the data from the plurality of the receiving stations and synthesizes the received data. The probability that a plurality of receiving stations geographically separated is all in rain at the same time is lower than the probability that a single receiving station is in rain. Thus, using the above-described site diversity, the probability that the signal attenuation occurs due to rain can be reduced, as compared to the case where the single receiving station is used. Consequently, using the above-described site diversity, the probability that the central station can receive the data correctly, that is, the probability that received data agrees with the data transmitted from the transmitting station becomes higher than the probability that the single receiving station, the receiving station can receive data correctly by using a single receiving station. When the above-described site diversity is used, the capability of the correct reception means that the data synthesized by the central station is correct data.
Adaptive modulation is the scheme to change a modulation scheme, a code rate, and the like of transmission signals in accordance with received signal quality. The adaptive modulation is a method referred to as adaptive coding and modulation (ACM) or variable coding and modulation (VCM) in Non Patent Literature 2, for example. This method performs the control to reduce a modulation level and a code rate when reception quality is low, and increase the modulation level and the code rate when the reception quality is high. This control enables transmission and reception satisfying a desired error rate.
As described above, the use of the site diversity can reduce the probability that the signal attenuation occurs due to rain or the like, as compared to the case where the single receiving station is used. This reduces the amount of rain attenuation that should be estimated in the channel design, and thus increases a design value in the received signal quality. Consequently, the modulation level and the code rate can be set higher to improve throughput.